Add List.asMap().

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index a11c59e45363e68bd0aa9a0d70434ec9ebd2cfa9..eb7bc70760ff48b847726524943831e8409efbb8 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -235,6 +235,10 @@ class _ObjectArray<E> implements List<E> {
   Set<E> toSet() {
     return new Set<E>.from(this);
   }
+
+  Map<int, E> asMap() {
+    return IterableMixinWorkaround.asMapList(this);
+  }
 }
 
 
@@ -474,6 +478,10 @@ class _ImmutableArray<E> implements List<E> {
   Set<E> toSet() {
     return new Set<E>.from(this);
   }
+
+  Map<int, E> asMap() {
+    return IterableMixinWorkaround.asMapList(this);
+  }
 }